Selective hydrogenation of acetylene to ethylene: Performance of a Pt monolayer over an α-WC(0001) surface for binding and hydroconversion of acetylene

Abstract

Ethylene (C2H4) is a useful hydrocarbon in polymerization reactions and as a probe system to understand hydrogenation reactions of complex unsaturated hydrocarbons and olefins. However, C2H4 streams usually contain small amounts of acetylene (C2H2), which deactivates the catalyst used for ethylene hydroconversions. This issue is overcome by hydrogenating C2H2 selectively into C2H4, avoiding further ethylene hydrogenation reactions. For this process, expensive palladium-based catalysts have shown good performance. Here, based on the results of density functional calculations, we propose the use of cheap and easily available tungsten carbide (WC) as a support of Pt, being an alternative material to Pt-group metals. Clean α-WC(0001)-C and α-WC(0001)-W were compared with Pt-supported on them, i.e. Pt/α-WC(0001)-C and Pt/α-WC(0001)-W. The theoretical results indicate that among the evaluated systems, the Pt/α-WC(0001)-W surface has a remarkable capacity to achieve selective hydrogenation of C2H2 into C2H4, with a reaction energy of -0.44 eV, avoiding further hydrogenation into ethyl (C2H5, +0.29 eV) and ethane (C2H6, +0.33 eV). In Pt/α-WC(0001)-W, the surface poisoning is avoided since ethylidyne (CCH3), a species responsible of catalyst deactivation is not formed. In contrast, the selective acetylene hydrogenation is not feasible on Pt/α-WC(0001)-C, α-WC(0001)-C, and α-WC(0001)-W; these surfaces are all poisoned due to the formation and deposition of CCH3 and C2H2 on them. The atomic charges indicate that the electron density flux from the Pt/α-WC(0001)-W surface to the C2H2 and C2H4 molecules is higher as compared to bare α-WC(0001)-W, since the Pt monolayer modulates electron density migration, as verified by a Projected Density of States (PDOS) analysis. The C2H4 desorption rate from Pt/α-WC(0001)-W is reasonable in the temperature range from 340 to 640 K, providing a theoretical basis for further practical catalysis. The results of this work show that Pt/α-WC(0001)-W is a good candidate for acetylene selective hydrogenation, opening a new window for further experimental and/or theoretical works.